Originally Posted by
chorlton
Food for thought.
Some time ago I wrote a program to calculate spoke lengths and it did the job but I also went and included something that gave the predicted spoke tensions based on the shortest spoke having the highest tension picking 130Kg as a base figure.
Recently I tacoed the wheels I had previously built. When I had finished them they did seem a bit tight when plucked but over time despite using stainless Alpina butted spokes they had become speckled by rust so time to build some new ones.
This time I invested in a Park Tools TM-1 and took the tension figures from the program as an initial target then laced the wheels to the point that the threads were just disappearing into the nipples then went around bringing the wheel up to tension.
When I had something stiff but not too stiff I worked around gradually bringing each spoke up to the tension suggested by the program.
In the front wheel this was,
Outer Right 113
Inner Right 130
Inner Left 130
Outer Left 113
In the rear wheel this was,
Outer Right 102
Inner Right 130
Inner Left 78
Outer Left 67
Whilst doing this every so often I worked around the pairs of spokes giving them a firm squeeze. When I had finished both wheels were basically true but needed some minor horizontal adjustment to true them fully. I should mention that this was with new rims and spokes.
Since a bad workman always blames their tools I will mention that the TM-1 seems to suffer from backlash so I had to make multiple squeezes on the tool to settle to a particular value. Also the relative precision or accuracy of the reading might be in doubt.
However given the marked different tensions as indicated by the program it seemed that the tool would give a good initial indication of the targets. At the end of the day, on a sample of two wheels, it would seem that the method used appears to work quite well.
Thoughts?
1. Each pairs of spokes on each side in a typical built wheel has two offsets. The difference will be 3.2mm - the typical thickness of an alloy hub. Given the same balanced nipple turn treatment, they will have a tendency to show different readings.
This is why after the initial tensioning of a wheel, I do a tension balancing treatment of each side of the wheel before continuing - to try to get ahead of this. I'll do it again later on in the build.
2. The TM-1 is a cheap budget tool. It works...sometimes it slips or as you metioned - "backlash". But so long as you keep it calibrated - roughly every dozen wheels - it works just fine.
3. All tension meters give estimates - NOT a precise or accurate reading.
Hence why I tell customers something like: "Drive side tension is between 95 kgf and 107 kgf."
4. Best us of a tension meter - calibrated or not - is to check relative tension.
That's how you verify that one noodle you see popping in or out while the wheel is spinning is not with the program.
I have a very old TM-1 that is set with a new version TM-1 spring. It simply cannot be calibrated according to the charts. So I just use it to check relative tension.
So word to the wise - don't toss out old tension meters. They still have other uses.
=8-|